This invention relates generally to acoustic well logging and more particularly to the detection of flow leaks through or behind the casing of a well penetrating the subsurface formations.
Acoustical noise logging of wells to determine the location of fluid flow thereinto is well known in the art. For example, in U.S. Pat. No. 2,210,417 to Kinley, leaks through casing are located by determining the location of sound produced by liquid passing through openings in the casing. A sound detector is moved through a well and is connected to an uphole indicating device or recording means. The intensity of sound produced by liquids passing through the casing is thus indicative of leaks in the casing, and location of such leaks is readily discernible from a graphical record of intensity versus the depth of the sound detector within the well. A similar method of determining the location of fluid flow into a well is disclosed in U.S. Pat. No. 2,396,935 to Wahlstrom.
A noise-logging technique for distinguishing single-phase flow from two-phase flow leaks through or behind the casing leaks is described in "The Structure and Interpretation of Noise From Flow Behind Cemented Casing," by R. M. McKinley, F. M. Bower, and R. C. Rumble, JOURNAL OF PETROLEUM TECHNOLOGY, March, 1973, pp. 329-338. The source of the leak is located from a noise-amplitude log and the type of leak (single- or two-phase flow) is determined from a spectrum of the noise source. A single-phase flow leak is one in which water is throttling into water or air is throttling into air. A two-phase flow is one in which air is throttling into water. The above-described article presents experimental results indicating that two-phase leaks have a noise-frequency structure distinguishable from that for single-phase leaks by a larger noise level in the band of 200 to 600 hertz. At higher frequencies, both single- and two-phase leaks have a noise characteristic of free-stream turbulence and are indistinguishable above 1,000 hertz.